1. Field of the Invention
This invention relates to a wave compression turbocharger assembly and more particularly to maintaining a close tolerance seal at the end faces of the rotor.
2. Description of the Prior Art
A wave compression turbocharger is a device for producing an exchange of pressure between the high energy state exhaust gas of an internal combustion engine and atmospheric pressure. Within the turbocharger, the ambient air is compressed and the exhaust gases expanded. A conventional wave compression supercharger includes a cylindrical rotor having radially directed vanes extending from its outer surface. Stationary port plates positioned at opposite ends of the rotor have openings formed through their thicknesses to allow exhaust gas and ambient air to flow into the rotor. Additional openings are provided in the port plates through which the expanded exhaust gas and compressed air flow from the rotor. The pressure exchange takes place within the rotor cells defined by the spaces between the rotor vanes.
The process of compressing the ambient air begins when a rotor cell rotates into alignment with the inlet port, thereby allowing ambient air to flow into and to fill the cell. Rotation then brings the rotor cell into alignment with the exhaust gas inlet port thus admitting a compression wave into the cell which begins to travel along the rotor length in the direction of the ambient air inlet. The exhaust gas temperature can be approximately 1600 degrees Farenheit. The compression wave travels along the rotor ahead of the engine exhaust gas and operates to compress the air in the rotor cell as it travels axially toward the air port plate. The rotor will have rotated out of alignment with the air inlet port when the compression wave has begun to travel down the rotor length and the air side of the rotor cell will have been sealed off by the air port plate.
Immediately before the compression wave reaches the air side of the cell, the cell rotates out of alignment with the exhaust gas inlet port. Next, the rotor brings the cell to the air outlet port thus allowing the compressed air to flow from the rotor due to the action of the compression wave traveling toward the air side of the rotor. The rotor then brings the cell out of communication with the air outlet port and for a brief period of time the rotor cell is closed off at both ends.
It is important to the efficient operation of a compression wave turbocharger that the rotor cells must be sealed tightly during operation. For example, during the compression portion of the rotor cycle while the compression wave is travelling from the gas side to the air side of the rotor, the rotor cell must be sealed at its axial ends to produce optimum compression of the ambient air. If the exhaust gas end of the rotor were not sealed adequately the compression wave would not travel completely along the rotor length to the air port plate and the efficiency of the device would be appreciably reduced.
In a similar way, the exhaust gas is purged from the rotor cell after the pressure wave rebounds from the air port plate surface. The rotor cell rotates into alignment with the exhaust gas outlet port when the rebounding compression wave returns to the exhaust gas port plate. At the air side of the rotor, the cell is opened to ambient air during the latter portion of the compression wave movement to the gas side so that a partial vacuum tending to resist movement of the turning compression wave is not produced.
It can be seen from this description of the sequence of events within the turbocharger that one axial end of the rotor is continually exposed to the high engine exhaust gas temperatures and that the opposite end of the rotor is kept at a relatively low temperature by the ambient air to which it is continually exposed. Therefore, the rotor has a substantial temperature gradient along its length. Because of the substantial heating of the rotor during operation, the rotor and the housing of the turbocharger experience thermal strain which produces a lengthening of the rotor as compared to its nonoperating condition.
Attempts have been made in the prior art to seal the ends of the rotor by providing a ceramic seal closely fitting the end faces of the rotor when the turbocharger is at ambient temperature. During operation the thermal growth of the rotor causes its end faces to cut into the ceramic seal thus maintaining a closely fitting seal at the elevated temperature. Examples of this means for producing a close tolerance seal between the rotor and the adjacent stator elements is described in U.S. Pat. Nos. 3,975,165 and 4,083,650. With this approach, after the ceramic seal has been formed with a depression caused by the axial growth of the rotor upon cooling, the rotor withdraws from close proximity to the seal, leakage is enhanced and low efficiency results at all temperatures less than the maximum operating temperature at which the depression had been formed. It is preferable for the rotor to be fitted with a close tolerance seal throughout its full operating temperature range. This invention is an improvement in wave compression turbo-chargers having ceramic rotors, examples of which are described in U.S. patent applications Ser. No. 32,198 and 32,324 both filed Apr. 23, 1979, now U.S. Pat. Nos. 4,269,570 and 4,274,811, respectively.